MEMS: The Biggest Word in Small

1 year ago

What’s tiny and on track to be worth $22 billion dollars by 2018? MEMS (Micro Electrical Mechanical Systems). That’s a catch-all phrase for microscopic devices that have moving parts. Usually, the component sizes range from 0.1 mm to 0.001 mm, which is tiny, indeed. There are some researchers working with even smaller components, sometimes referenced as NEMS (Nano Electrical Mechanical Systems).

MEMS have a wide range of applications including ink jet printers, accelerometers, gyroscopes, microphones, pressure sensors, displays, and more. Many of the sensors in a typical cell phone would not be possible without MEMS. There are many ways that MEMS devices are built, but just to get a flavor, consider the cantilever (see right), one of the most common MEMS constructions.

Cantilever

In mechanical engineering, a cantilever is a rigid structure, often a beam or a plate, anchored at one end to a support. Any load applied to the cantilever transmits to the support. This is often used when building bridges, for example.

MEMS devices widely use cantilevers at the microscopic level. These can act as transducers for atomic force microscopes, or as resonant elements in filters and resonators. Cantilevers can also act as accelerometers. On the left, you can see a microscopic cantilever vibrating in resonance.

It is relatively straightforward to detect acceleration using a cantilever. First, you attach a microscopic proof mass to the end. Acceleration will cause that mass to move, thus stressing the cantilever. By measuring the stress on the support, you can determine the amount of acceleration force on the cantilever. By positioning cantilevers on different axes, you can read acceleration in each direction.

History

While the term itself dates back to 1986, the idea is much older. In 1959, for example, Richard Feynman lectured about the possibility of such devices. However, practical construction using semiconductor manufacturing techniques made the devices theoretical for a long time.

Even so, Feynman’s lecture anticipated a few key points to MEMS and even created two challenges. One was to construct a tiny motor and was completed using conventional tools in 1960. The other challenge had to wait until 1985 when a graduate student reduced a passage of text to be 25,000 times smaller.

Unfortunately, Feynman was ahead of his time, and building a tiny motor conventionally didn’t really advance the state of the art. One of Feynman’s key points that you could make a set of remote manipulators at, say, quarter-scale. Then you could use those manipulators to build another set at 1/16th scale and keep repeating the process. Feynman knew that you’d eventually have to change how the manipulators work because materials behave differently at scale and forces that act on things get funny as the scale goes down, too.

Future

Of course, the cantilever is just one possible device. There are MEMS temperature sensors, magnetic field sensors, radiation sensors, and more. There are microscopic motors that use electrostatic force instead of electromagnetic, micro gas valves, and optical switches.

Because the MEMS devices use semiconductor fabrication techniques, it is inviting to integrate them with conventional circuits. We are already starting to see microcontrollers with MEMS devices onboard and we expect to see that trend continue.